The present invention relates to a semiconductor packaging metal lid for use in hermetically sealing a semiconductor element mounted on an insulating substrate.
A class of hermetically sealed semiconductor packages is manufactured by placing a sheet- or cap-like lid over a semiconductor element mounted on an insulating substrate such as a ceramic substrate so as to enclose the semiconductor element and by subsequently joining peripheral sealing areas of the substrate and the lid with a sealer for hermetic sealing. These packages are primarily employed in applications where thermal resistance and a high level of durability and reliability are required. The sealer may be soft solder for soldering, hard solder for brazing, or glass. Of these materials, soft solder (which is often referred to merely as solder) is lowest in melting temperature and is advantageous in that sealing can be performed at a relatively low temperature.
The lid is usually made of a ceramic or metallic material. In the case of sealing by soldering with soft solder, a ceramic lid must be metallized to form a thick metal layer on the peripheral sealing area of the lid to enable the lid to be bonded by soldering, which adds to the production costs of the lid. In contrast, a metal lid can be directly bonded by soldering without such a metallizing procedure and is advantageous from an economic viewpoint.
A typical metal lid known in the art comprises a lid body comprising a metal plate of a predetermined shape having a layer of solder serving as a sealer only in the peripheral sealing area on one surface of the lid body. The solder employed for this purpose is a class of soft solder called high-temperature solder which has a melting temperature around 300.degree. C. Examples of such solder includes Pb/Sn alloys and Pb/Sn/Ag alloys.
The metal lid having the solder layer in the peripheral sealing area is placed on a substrate (on which a semiconductor element has been mounted) such that the peripheral solder layer of the lid faces and contacts a peripheral sealing area of the substrate, with the semiconductor element being enclosed in the space formed between the lid and the substrate. Then, the assembled substrate and lid are heated by an appropriate means such as an oven at a temperature sufficient to melt the solder in order to join the lid to the substrate, thereby forming a hermetic seal in the sealing area and producing a hermetically sealed semiconductor package.
Such a packaging metal lid for use in hermetic sealing, which comprises a lid body and a solder layer for sealing in a peripheral sealing area, has been produced either by (1) applying a solder paste comprising a solder powder and a flux to the peripheral sealing area of the lid body by printing followed by reflowing to evaporate the flux and form a solder layer, or (2) preforming a solder sheet in the form of a rectangular annulus (corresponding to the shape of the sealing area) by punching from a blank sheet of solder, placing the preformed solder sheet in position on the lid body, and securing it to the solder sheet by spot welding in several locations.
Process (1) which is conducted by printing with a solder paste is less expensive and is more readily adaptable to mass production than Process (2) which involves punching of a blank solder sheet and spot welding. However, Process (1) often causes part of the flux in the solder paste to remain in the solder layer after reflowing, thereby forming voids in the solder layer. The residual flux entrapped in the voids may vaporize during heating to melt the solder layer for hermetic sealing and cause the voids to burst, which in turn causes the molten solder to splash around in the form of solder balls, which may be deposited on the interconnecting lines and semiconductor element on the substrate. The deposited solder balls have a diameter which is as small as from about 3 .mu.m to about 5 .mu.m, for example, but they may nevertheless seriously impede the performance of the semiconductor element.
In Process (2), the above-mentioned problem attributable to the residual flux is eliminated. However, as the blank solder sheet is too soft to be handled easily, the manufacturing operations which includes punching the blank solder sheet into a preformed solder sheet having a rectangular annular shape, placing the preformed solder sheet in position on the lid body, and securing it to the lid body by spot welding involve many difficulties and greatly interfere with the productivity of the process.
Furthermore, as pointed out in Published Japanese Patent 2,550,667, spot welding is performed by pressing an electrode of a spot welder against the solder sheet, which causes the solder sheet in the pressed spot to sink and the surrounding area of the solder sheet to rise, thereby making the solder sheet bumpy. The bumpiness of the solder sheet may lead to misplacement of the lid on the package substrate before hermetic sealing of the package, or may adversely affect the dimensional accuracy of the resulting semiconductor package due to the uneven thickness of the solder layer. Moreover, the tightness of the hermetic seal formed by the bumpy solder layer may be degraded due to injury by the raised portions or retention of air in the depressed portions.
In order to cope with these problems, it is proposed in Published Japanese Patent 2,550,667 to secure the solder sheet to the lid body by a non-contact heating means capable of localized heating such as a laser beam, instead of by spot welding. As a result, the above-described problems attributable to the bumpiness of the solder sheet can be eliminated. However, the problem of low productivity due to the poor handling properties of the soft solder sheet remains unresolved. Therefore, Process (2) is less adaptable to mass production than Process (1).